Conventionally in this type of image formation apparatus, there is known a method for forming a developing nip by making a developer support which supports a thin film of a liquid developer on the surface thereof abut against a latent image support, and developing the latent image formed on this latent image support using the liquid developer. For example, in Japanese Patent Application No. 11-38447, the present applicant has proposed an image formation for forming a nip section by making a developer support having an elastic layer abut against a latent image support.
In this image formation method, a thin film of a liquid developer is formed on a developer support, and a toner in the thin film is electrostatically transferred to an image section in the latent image on the latent image support which forms the nip section, to thereby effect development. On the other hand, the toner is not allowed to adhere on the ground section (on the background section) on the latent image support which passes through the nip section.
FIGS. 2A and 2B are schematic diagrams which show the condition of the developer 60 at the developing nip. FIG. 2A shows the developing nip between an image section on a photosensitive drum 1 and a developing roller, and FIG. 2B shows the developing nip between the background section on the photosensitive drum and the developing roller. Prior to entering into the developing nip, the toner density in the developer layer is substantially uniform, but when the toner enters into the developing nip, the toner starts to migrate in the developer layer, and as the toner proceeds in the developing nip, the toner density in the developer layer has a gradient. As shown in FIG. 2A, in the image section, the density of the toner 60a in the developer becomes such that it becomes relatively high on the photosensitive drum 1 side and relatively low on the developing roller side. Also as shown in FIG. 2B, in the background section, the gradient of the toner density becomes opposite to that in the image section. Therefore, in the developing nip, it is necessary to secure the developing time (the time for the thin layer of the liquid developer to pass via the nip) so that the toner can electrically migrate sufficiently. By securing sufficient developing time, sufficient development is performed, and high image density contrast (high image density, low ground density), high resolution and excellent uniformity in contact print can be obtained.
The developing time relies on the width of the developing nip (the size of surface migration on the photosensitive drum and the developing roller at the developing nip, and hereinafter referred to as “width of the developing nip”), and the process linear velocity which is the peripheral speed of the latent image support and the developer support. Normally, by setting the width of the developing nip to at least a value obtained by multiplying the process linear velocity by the developing time constant, such developing time can be secured. This developing time constant is a time required for the developed amount to saturate, and is a value obtained by dividing the process linear velocity by a minimum width of the developing nip required for the saturation of the developed amount. For example, if the process linear velocity is 300 mm/sec, and the developing time constant is 10 msec, the width of the developing nip becomes 3 mm.
When the width of the developing nip is too small compared to a predetermined size, sufficient development cannot be performed at the developing nip, and the density of the toner image becomes low. On the other hand, when the width of the developing nip is too large compared to the predetermined size, sufficient density of the toner image can be obtained, but the toner may adhere on the ground section of the latent image support, to thereby cause a phenomenon referred to as fogging (also referred to as greasing). Therefore, the width of the developing nip is set to an optimum value, taking the process linear velocity and the developing time constant into consideration in the designing step.
In Japanese Patent Application No. 11-38447, the applicant of this invention has proposed an image formation method for forming a nip section by making a developer support which has an elastic layer abut against a latent image support. In this image formation method, a thin layer of a liquid developer is formed on the developer support, so that the carrier liquid in the thin layer and the toner are electrostatically transferred to an image section in the latent image on the latent image support which forms the nip section, to thereby effect development. On the other hand, the toner is not allowed to adhere on the ground section (on the background section) on the latent image support which passes through the nip section, and only the carrier liquid is slightly transferred thereto.
Even if the toner adheres on the ground section, it is possible to shift the toner towards the developer support to thereby remove it, while passing through the nip section.
In order to prevent residual toner, there is also a technique which prevents adhesion of the toner onto the background section by forming a sufficient electric field between the background section on the latent image support and the developer support (hereinafter referred to as a background section developing electric field). In the method of preventing the toner from adhering on the background section by this background section developing electric field, the larger the background section developing electric field, the larger the effect of prevention of the toner adhesion onto the background section.
The present applicant has also proposed a method of removing the residual toner remaining in the background section after development by a removal member, in Japanese Patent Application No. 2000-42582. Specifically, an electric field (hereinafter referred to as removal electric field) is formed between the background section and the removal member, to attract the floating residual toner towards the removal member to thereby remove the residual toner from the surface of the latent image support. By this proposal, image fogging due to the residual toner is prevented.
In liquid developing apparatus of an electrostatic latent image which develops an image by a toner, as a method of supplying a liquid developer to the latent image face on an image support, there have been used a method for providing unevenness on the surface of the developing roller which is developer support, and holding a liquid developer in the recess to supply it to the image support, a method for using a sponge roller as the developer support, and supplying the liquid developer absorbed by the sponge roller to the image support by pressing the sponge roller against the image support, a method for supplying the liquid developer to the image support directly, without using the developer support, by soaking the image support in a developer tank which stores a liquid developer.
However, the nip width determined at the time of designing may not be formed as designed in the designing step after completion of the apparatus assembly, due to the influence of the production accuracy and assembly accuracy of the parts. By improving the production accuracy and assembly accuracy of the parts, the width of the developing nip can be formed within the allowable range in the design, but it may cause an increase in the production cost or the structure may become complicated.
If it is tried to increase the image formation speed by using the image formation method proposed in Japanese Patent Application No. 11-38447, the developing speed may not catch up with the speed to thereby cause insufficient image density, or the density in the ground section may become excessive to thereby cause image fogging.
When it is desired to increase the image density, there is a method for increasing the amount of developer to be applied on the developer support. However, if the amount of developer to be applied on the developer support is increased, the distance between the developer support and the latent image support (developing gap) increases, to decrease the electric field. Thereby, there is a problem in that the developing speed cannot catch up with the speed to thereby cause insufficient image density or image fogging.
Depending on the surface smoothness of a transfer material, the amount of developer to be applied on the developer support may be too much, thereby the toner image may collapse, or the image density may be too high. Therefore, when the unevenness on the surface of the transfer material is relatively small, and the surface has excellent smoothness, an excellent image can be obtained by reducing the toner layer in the toner image to be transferred, as compared with when the unevenness on the surface of the transfer material is relatively large, and the surface has poor smoothness. Therefore, a requirement for image density at the time of transfer is different depending on the transfer material to be used.
Hence, it is desired to change the width of the developing nip to a desired size depending on the circumstances.
When the developing nip is formed, it can be formed easier when the elastic layer on the developer support has a low hardness than the instance of having a high hardness, by elastically deforming the elastic layer with a small pressing force, and hence, the load on each member can be reduced, and the durability of the apparatus can be improved. In order to produce an elastic layer of a low hardness, normally oil is contained. However, the elastic layer containing oil has a problem in that the oil begins to leak at the time of use to thereby affect the formed image, or the elastic layer may shrink due to leakage of the oil. The elastic layer containing oil has another problem in that it absorbs the liquid developer or its component and swells with the lapse of time.
In the image formation method proposed in Japanese Patent Application No. 11-38447, the toner adheres on the ground section (background section) on the latent image support which is passing through the nip section, which may remain as a residual toner. In this instance, this residual toner may cause image fogging. Particularly in an instance of a developer having high solid (the toner and other resins, etc.), this tendency becomes conspicuous.
In the method of removing the floating residual toner by forming the removal electric field, as the background section developing electric field increases, a force of pressing the residual toner in the background section against the developer support by the electric field also increases. If the background section developing electric field is excessively increased in order to prevent toner adhesion onto the background section, there has been heretofore a problem in that the toner pressed against the developer support may flocculate on the developer support.
In the method of preventing image fogging by attracting the residual toner towards the removal member by the removal electric field, described below, there is such a problem that as the removal electric field is increased, the residual toner attracted towards the removal member may flocculate.
Thus flocculated residual toner has a large particle diameter, and when this toner is reused for development, reproducibility of fine lines is poor. Therefore, it is desired not to cause flocculation of the residual toner.
In the conventional image formation apparatus, at the time of development, if development is insufficient, the toner is unevenly distributed in a stripe form (ribs), and hence the image does not have a uniform density. In the image formation apparatus using this image formation method, a carrier liquid which is nonvolatile at a normal temperature and has high viscosity is used so that the carrier liquid does not diffuse into the air, taking the environment into consideration, and the liquid is not likely to scatter, taking handling into consideration. The high-viscosity carrier liquid adheres onto transfer paper in a larger amount than the low-viscosity carrier liquid, and there is a problem in that if the carrier liquid adheres on the transfer paper in a large amount, the appearance and touch of the transfer paper changes from the original paper.
With the conventional electrostatic recording apparatus or the like, when a liquid developer is supplied to an image support, a low-viscosity liquid developer is normally used, in which a toner is mixed in IsoparG (registered trademark, manufactured by Exxon Co.) which is an organic solvent at a rate of about 1 to 2%. In order to realize safe and small liquid developing apparatus by suppressing steam generation of the solvent, however, it is desired to use a high-viscosity liquid developer having higher density than the liquid developer used for the conventional apparatus. However, liquid developing apparatus that can supply a developer having stable toner density to an electrostatic drum has not yet been proposed. In the technique disclosed in Japanese Patent Application Laid-Open No. 8-328392, a plurality of rollers is used to control the thickness of a developer layer. However, with this technique, the developing space between an image support and a developer support changes, thereby stable development cannot be performed, and a unit which controls the density of the developer is not described therein. Therefore, when such a high-density and high-viscosity liquid developer is used, it is not clear which method is suitable as a method of supplying a liquid developer to a latent image face on the image support.